While not limited thereto, the present invention is particularly adapted for use with electronic balancing equipment and other similar apparatus where it is necessary to manually tune a filter until a mark on a rotating body to be balanced appears frozen under the influence of a strobe lamp and is indicating the correct phase, or until the indicated amplitude of the incoming vibration signal reaches a peak, indicating that the filter is properly tuned to the frequency of a single vibration signal. The purpose of this is to insure that only vibration signals having a frequency corresponding to the rotational speed of the body to be balanced enter the phase or amplitude indicating or strobe apparatus. Particularly in those cases where the vibration amplitude is relied upon and is indicated on a digital display, it is somewhat difficult for an operator to get the "feel " of whether the filter frequency is on one side or the other of the correct frequency indicated by a maximum signal amplitude. Of course, there are many other applications, other than vibration analyzing apparatus, where fine tuning of a filter or controlled element must be accomplished.